Laser satellite communication system

ABSTRACT

A laser communication system adapted for use in a satellite communication system. The satellite carries a laser communication system. The laser communication system includes a plurality of active and passive optical elements packaged in a monolithic, or single block, structure for interfacing between a focusing beam director of the satellite and laser transmitters/receivers of the laser communication system. Laser energy is directed between the beam director and the transmitters/receivers by the active and active optical elements, such laser energy passing through the monolithic structure solely as collimated light. In this way, relay elements, such as diffractive optics and focusing lenses, and there concomitant alignment requirements, are eliminated from the monolithic structure.

This is a divisional of copending application Ser. No. 08/395,452, filedFeb. 28, 1995.

BACKGROUND OF THE INVENTION

This invention relates generally to satellite communication systems andmore particularly to laser satellite communication systems wherein datais transmitted to, and/or from a satellite using lasers.

As is known in the art information is sometimes transmitted betweenvarious locations on the earth by routes which include satellites. Moreparticularly, in the routing process, information may be transmittedfrom a ground station along the route to a satellite. The receivingsatellite may, in some arrangements, retransmit the information to aremote ground station along the route. In other arrangements, thereceiving satellite may be retransmit the information directly toanother satellite along the route, which, in turn may itself retransmitto another satellite, or to a remote ground station. The transmissionpath, or data link, directly between a pair of satellites is sometimesreferred to as an inter-satellite cross link in the routing process.While transmission of data between the ground station and satellite istypically by radio frequency (RF) energy, the use of laser energy, atleast for communication between satellites (i.e., for the intersatellite cross links) offers distinct advantages over radio frequency(RF) systems, particularly for satellite cross links. These advantagesinclude the potential for a great reduction in weight, power for a givendata rate, lack of optical spectral congestion and frequency allocationrequirements, immunity to electromagnetic interference, co-locatedtransmitters and RF jammers.

SUMMARY OF THE INVENTION

In accordance with the present invention, a laser communication systemadapted for use in a satellite communication system is provided. Thesatellite carries a laser communication system. The laser communicationsystem includes a plurality of active and passive optical elementspackaged in a monolithic, or single block, structure for interfacingbetween a focusing telescope of the satellite and lasertransmitters/receivers of the laser communication system. Laser energyis directed between the telescope and the transmitters/receivers by theactive and passive and optical elements, such laser energy passingthrough the monolithic structure solely as collimated light. In thisway, relay elements, such as diffractive optics and focusing lenses, andtheir concomitant alignment requirements, are eliminated from themonolithic structure.

In accordance with another feature of the invention, the monolithicstructure is configured to provide all optic axes between the telescopeand a laser transmitter/receivers in substantially a common plane. Moreparticularly the laser communication system includes an acquisitionlaser transmitter and an acquisition receiver used to enable thesatellite to link up with another satellite, or ground station, duringan acquisition mode, and a communication laser transmitter and ancommunication receiver used to enable the satellite to exchange datawith the linked up satellite, or ground station. The monolithicstructure is configured to dispose the optic axes between the telescopeand laser acquisition and communication lasers and the optic axesbetween the telescope and the acquisition and communication receivers insubstantially a common plane. With such an arrangement, the structuralrigidity and hence optical integrity of the monolithic structure isimproved.

In accordance with an additional feature of the invention, a singledetector is provided for both the acquisition mode and a subsequenttracking mode. More particularly, the laser communication systemincludes a tracking laser transmitter and a tracking receiver used toenable the satellites to track each other during the tracking mode andthereby maintain the link up with the other satellite, or groundstation, after the above described acquisition mode. The satellitescommunicate with one another during the tracking mode. In a preferredembodiment of the invention, a single charge coupled device (CCD) isused for both acquisition and tracking.

In accordance with still another feature of the invention, acollimating/beam shaping module is provided having affixed thereto apair of submodular units. More particularly, as noted above, the laser'slight passes through the monolithic structure solely as collimatedlight. In passing between a laser in the system and the monolithicstructure, the laser's light beam must shaped and collimated. The firstsubmodular unit includes the laser and a properly aligned beam shapinglens. The second submodular unit includes a mounted collimating lens.The first and second submodular units are aligned with each other andthen affixed to each other to provide the collimating/beam shapingmodule. Next, the collimating/beam shaping module is affixed to themonolithic structure. With such arrangement and method proper accuratealignment of the mounted laser, beam shaping lenses and collimating lensis facilitated.

In accordance with still another feature of the invention, a filter isprovided on a surface of the second submodular unit. The filterprotrudes beyond the second submodular unit and is provided with asurface adapted to interface, and be affixed to, a surface portion ofthe monolithic structure.

BRIEF DESCRIPTION OF THE DRAWING

For a more complete understanding of the concepts of the invention, aswell as the invention itself, reference is now made to the followingdrawings, in which:

FIG. 1 is a sketch of a satellite communication system wherein a pair ofsatellites communicate with each other using an inter-satellite crosslink routing process, each one of such satellite carrying a lasercommunication system according to the invention;

FIG. 2 is a plan view of the laser communication system used in thesatellite communication system of FIG. 1;

FIG. 3 is a cross section, elevation view of a laser transmitter moduleused in the laser communication system of FIG. 2;

FIG. 4 is a cross section view of the laser transmitter module of FIG.3, such cross section being taken along line 4--4 in FIG. 3;

FIG. 5 is a cross sectional elevation view of a submodular units of themodule of FIG. 3, such cross section being taken along line 5--5 in FIG.4;

FIG. 6 is a diagram useful in understanding a tracking system used inthe laser communication system of FIG. 1;

FIG. 7 is a block diagram of a control system used in the trackingsystem of FIG. 6; and

FIG. 8 is a diagram useful in understanding the tracking system of FIG.6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, a laser satellite communication system 10 isshown. Here, a ground station 12 transmits information to an orbitingsatellite 14, the satellite 14 receives the information and relays it,via a laser communication system 16, to be described in detailhereinafter, on board the satellite 14, to a second orbiting satellite18 in an inter-satellite cross link in the routing process. The secondorbiting satellite 18, also carries a laser communication system, notshown, such as the system 16 carried by satellite 14, and then transmitsthe information to a second ground station 20, as shown. Here theinformation is transmitted between the satellites 14, 18 and the groundstations 12, 20 using radio frequency signals. It should be understoodhowever that laser (i.e., light) energy signals may also be used.Further, while only two satellites 14, 18 have been shown, many moresatellites may be cross linked; either in a low earth orbitconstellation (LEO) or in a geosynchronous orbit (GEO) constellation.

Referring now to FIG. 2, the laser communication system 16 disposed ineach one of the satellites 14, 18 includes: a pair 22 of acquisitionlaser transmitters 22a, 22b; a pair 24 of tracking laser transmitters24a, 24b; a pair 26 of communication laser transmitters 26a, 26b; anacquisition/tracking detector/receiver 28; a communicationdetector/receiver 30; and a beam director 33, here made up of a finesteering mirror 31 and a telescope 32, all optically coupled together ina manner to be described in detail hereinafter by a monolithic opticalstructure 34. Suffice it to say here, however, that the monolithicoptical structure 34 includes a plurality of active and passive opticalelements for interfacing between the beam director 33 and the lasertransmitters 22a, 22b, 24a, 24b, 26a, 26b and the detector/receivers 28,30. Laser energy is directed between the laser transmitters 22a, 22b,24a, 24b, 26a, 26b and the beam director 33 and between the beamdirector 33 and the detector/receivers 28, 30 by the active and passiveoptical elements. The laser energy passes through the monolithic opticalstructure 34 solely as collimated light. The monolithic opticalstructure 34 is configured so that all optic axes between the beamdirector 33, the laser transmitters 22a, 22b, 24a, 24b, 26a, 26b and thedetector/receivers 28, 30 are disposed in substantially a common plane,here the X-Y plane. The acquisition laser transmitters 22a, 22b and anacquisition/tracking detector/receiver 28 are used to enable thesatellite to link up with another satellite, or ground station, duringan acquisition mode. The tracking laser transmitters 24a, 24b are usedto maintain track with the other satellite, or ground station during asubsequent tracking mode. The communication laser transmitters 26a, 26band communication detector/receiver 30 are adapted to enable thesatellite to exchange data with the linked up satellite, or groundstation during the tracking mode. As noted above, the monolithic opticalstructure 34 is configured to dispose the optic axis between the beamdirector 33 and laser acquisition, tracking and communication lasertransmitters 22a, 22b, 24a, 24b, 26a, 26b and the optic axes between thetelescope 32 and the acquisition and communication detector/receivers28, 30 in substantially a common plane, here the X-Y plane.

As noted above, the laser transmitters 22, 24, 26 and detector/receivers28, 30 include: tracking laser transmitter 24 for enabling the linked upsatellite, or ground station to track the satellite during a trackingmode. A single acquisition/track, here (A/T) detector 28, is used by thesatellite during both the acquisition mode and the subsequent trackingmode. As will be described in detail hereinafter, the single detector 28uses a charge coupled device (CCD).

The laser communication system 16 includes for each one of the lasertransmitters 22a, 22b, 24a, 24b, 26a, 26b a collimating/beam shapingmodule 38. Each one of the module 38 will be described in detailhereafter in connection with FIGS. 3, 4 and 5. Suffice it to say here,however, that the module 38 includes a pair of submodular units 40, 42.A first one of the submodular units, here modular unit 40 includes oneof the transmitting lasers 22a, 22b, 24a, 24b, 26a, 26b, respectively,as shown, and a beam shaping lens 41, the second one of the submodularunits 42 having a collimating lens 44, as shown. The collimating lens 44is, here, a molded aspheric collimating lens. The first and secondsubmodular units 40, 42 are aligned with, and affixed to, each other toprovide the collimating/beam shaping module 38. The collimating/beamshaping module 38 is then affixed to the monolithic optical structure34. A bandpass filters 150, 152, 154, 156, 158 and 160 are disposed onan surface of the second submodular units 42, as shown. The bandpassfilter 150-160 protrudes beyond the second submodular unit 42 and isprovided with a surface adapted to interface, and be affixed to, asurface portion of the monolithic optical structure 34.

More particularly, the monolithic optical structure 34 includes aplurality of glass cubes and planar thin films bonded together anddisposed to pass such light signals therethrough as only substantiallycollimated light. The pair of acquisition laser transmitters 22 includea primary acquisition laser transmitter 22b and a redundant acquisitionlaser transmitter 22a. Here, both such acquisition laser transmitters22a, 22b transmit light at a predetermined wavelength, here 810 nm, butwhich can operate from 804 to 816 nm over temperature. The primaryacquisition laser transmitter 22b here transmits light with, here,vertical polarization; while the redundant acquisition laser transmitter22a transmits such light with, here, horizontal polarization. The pairof tracking laser transmitters 24 include a primary tracking lasertransmitter 24b and a redundant tracking laser transmitter 24a. Here,both such tracking lasers transmitters 24a, 24b transmit light at apredetermined wavelength, here also 810 nm. It is again noted thatbecause the acquisition and tracking laser transmitters 22, 24 use thesame wavelength, the laser transmitters 22, 24 operate sequentiallyrather than simultaneously. Here both the primary and redundant trackinglaser transmitters 24a, 24b transmit light with the same, herehorizontal polarization. The pair of communication laser transmitters 26include a primary laser transmitter 26b and a redundant communicationlaser transmitter 26a. Here, both such communication lasers transmitters26a, 26b transmit light at a predetermined wavelength, here a longerwavelength of 860 nm. Here, both the primary and redundant communicationlaser transmitters 26a, 26b transmit light with the same, herehorizontal polarization. The acquisition/tracking detector/receiver 28is adapted to operate with laser energy of, here, 780 nm while thecommunication detector/receiver 30 is here adapted to operate with laserenergy of 830 nm. Here, the telescope 32 is a Cassegrainian telescope ofany conventional design.

As noted above, the monolithic optical structure 38 includes thin films.One type of thin film used is, here a liquid crystal polarizationrotator. As discussed in my paper entitled "Technologies and techniquesfor lasercom terminal size, weight, and cost reduction" Free-Space LaserCommunication Technology II, Proc. SPIE, VOL. 1218, pp. 62-69, 1990, myco-authored paper entitled "Liquid crystals for laser applications" byChinh Tan and myself, Robert T. Carlson, published January 1991 inProceedings of the SPIE, #1866 as well as in my co-authored paperentitled "An advanced lasercom terminal for intersatellite crosslinks"by myself, Robert T. Carlson, Voula C. Georgeopolous, and Jerold L.Jaeger, published Mar. 2, 1994 in The Proceedings of the 15thInternational Communications Satellite Systems Conference, March, 1994,the material in all such papers being incorporated herein by reference,by applying a proper voltage on the liquid crystal film, here nematicliquid crystal, light of different polarizations may be directeddifferently. Here, for example, thin films of nematic liquid crystal athigh voltage provide no phase retardation to the light passingtherethrough with the result that the polarization of such light remainsunchanged. At minimum voltage, however, the polarization of lightentering the nematic liquid crystal is rotated 90 degrees so thatvertically polarized light entering the crystal leaves the crystal ashorizontally polarized light, on the one hand, and horizontallypolarized light entering the crystal leaves the crystal as verticallypolarized light, on the other hand. Thus, by placing a polarizationbeamsplitter in the path of the light leaving the liquid crystalrotator, vertically polarized light is, for example, reflected by thebeamsplitter to pass along one direction, horizontally polarized lightis transmitted through the beamsplitter to pass along another direction.Thus, light may be directed to pass in one of the two directions,selectively, in accordance with the electrical voltage (i.e., controlsignals) applied to the liquid crystal rotator. Thus, here, as will bedescribed in detail hereinafter, the thin films are responsive toelectrical control signals to direct the light signals from either thefirst primary laser transmitter 22b, 24b, 26b or the redundant lasertransmitter 22a, 24a, 26a through the monolithic optical structure 34 tothe beam director 33, selectively, in accordance with the controlsignals. Such is the case for each of the pair of laser transmitters 22,24, 26; i.e., the pair of acquisition laser transmitters 22 (i.e., theprimary acquisition laser transmitter 22b and the redundant acquisitionlaser transmitter 22a), the pair of tracking laser transmitters 24(i.e., the primary tracking laser transmitter 24b and the redundanttracking laser transmitter 24a) and the pair of communication lasertransmitters 26 (i.e., the primary laser transmitter 26b and theredundant communication laser transmitter 26a). Additionally, a liquidcrystal polarization rotator thin film is responsive to electricalcontrol signals to attenuate light from the sun, or signals from anothersatellite which may saturate either one of the detector/receivers 28,30. Here, the control signals are developed from the detector/receiversand provide a feedback signal to the thin film, as will be describedhereinafter.

Referring now to the details of the monolithic optical structure 34, itshould first be noted that the structure includes a plurality of, heretwelve glass cubes, here one-half inch glass cubes, which provide: fivepolarization beamsplitters (PBS) 50, 52, 54, 56, 58, two dichroicbeamsplitters (DBS) 60, 62, four folding mirrors 64, 66, 68, 70, and aspacer 72. Also included are a plurality of, here nine active thin film,nematic liquid crystal (LC) polarization rotators: two X-Y planealignment liquid crystal polarization rotators (LC X-Y PLANE ALIGN) 106,108 each responsive to control signals on lines 110, 112, respectively,two Y-Z plane alignment liquid crystal polarization rotators (Y-Z PLANEALIGN) 114, 116, each responsive to control signals on lines 118, 120respectively, and five 0/90 degree phase retardation liquid crystalpolarization rotators (LC POL ROT) 122, 124, 126, 128 and 130, eachresponsive to control signals on lines 132, 134, 136, 138, 140, Alsoincluded are eight passive optical wedges for course alignment (ALIGN)74, 76, 78, 80, 82, 84, 86, and 88. The course alignment wedges 74, 76,78, 80, 82, 84, 86, and 88 are, here, fused silica spacers polished tothe required wedge angle to cause course alignment in the X-Y and Y-Zplanes. Precision alignment is subsequently accomplished with the liquidcrystal X-Y plane and Y-Z plane alignment devices 106, 108, 114, and116. (It should be noted that the course alignment wedges 74-88 areadapted to deviate the beam, or optic axis, between one and tenmilliradians the X-Y plane and Y-Z plane alignment wedges 106, 108, 114,106 are adapted to deviate the beam, or optic axis, one milliradian, orless. Therefore, while deviations of up to about ten milliradians arepossible, all optic axes between the laser transmitters 24, 26, 28 andthe beam director 33 and between the beam director 33 and thedetector/receivers 28, 30 are substantially in a common plane. To put itanother way, all optical blocks, thin films and other optical elementsof the monolithic optical structure 34 are in a common plane, here theX-Y plane.) Also included is a plurality of, here nineteen, passive thinfilm devices: three half wave plates 142, 144, 146, one quarter waveplate 148, eight bandpass filters 150, 152, 154, 156, 158, 160, 162,164, and three polarization filters 166, 168, 170 used to purify thedesired linear polarization, two spacers 172, 174, to provide properalignment of the blocks, and two absorptive neutral filters (ND) 173,175. A heat sink 178 is mounted, as shown. The elements 50-58, 60-62,64-70, 72, 74-88, 106-108, 114-116, 122-130, 142-146, 148, 150-164,166-170, 172-175 are permanently bonded together with any suitableoptical cement, to form a monolithic optical structure 32, as shown; allsuch elements being disposed in a common plane, here the X-Y plane, asshown. Fused silica is used throughout because of its superb radiationresistance and thermal stability. The monolithic optical structure 34 ishere, less than 6 cubic inches (six inches along the X axis, 2.5 inchesalong the Y axis and 0.5 inches along the Z axis) and weighs less than 1pound. Because optical paths are short, all light passing through thestructure 34 is substantially collimated light thereby eliminatingrelay, or focusing optical assemblies and their concomitant alignmentrequirements. Further, a pair of lenses, beam shaping lens 41,collimating lens 44 are mounted within a single module 38, along withtheir associated laser transmitter, in a manner to be described indetail hereinafter in connection with FIGS. 3, 4, and 5. The lenses 41,44 are used for beam shaping and collimating, respectively, the lighttransmitted to the monolithic optical structure 34 by the lasertransmitters 22, 24, 26. Lenses 45, 47 are used for focusing thecollimated light exiting the monolithic optical structure 34 to thedetector/receivers 28, 30, respectively, as shown.

As noted above, light transmitted by either one of the communicationlaser transmitters 26a, 26b (i.e., entering the monolithic opticalstructure 34) is horizontally polarized. Likewise, light transmitted byeither one of the tracking laser transmitters 24a, 24b is horizontallypolarized. However, the wavelength of the light transmitted by thecommunications laser transmitters 26a, 26b and by the tracking lasertransmitters 24a, 24b are different. The wavelength of the lighttransmitted by either one of the tracking laser transmitters 24a, 24b isshorter than the light transmitted by either one of the communicationlaser transmitters 26a, 26b. Here, the wavelength of each one of thecommunication laser transmitters 26a, 26b is 860 nm and the wavelengthof each one of the tracking laser transmitters 24a, 24b is 810 nm.

The horizontally polarized light from the primary communication lasertransmitter 26b passes through the bandpass filter 158 to half waveplate 146 for conversion to vertical polarization. The verticallypolarized light passes through the course alignment wedge 82 and is thenreflected by the folding mirror 68 to a polarization beamsplitter 58.The polarization beamsplitter 58 reflects the vertically polarized lightto the liquid crystal polarization rotator 128. In the high voltagestate, the liquid crystal polarization rotator 128 passes the verticallypolarized to the Y-axis and X-axis alignment liquid crystal devices 116,108, to a dichroic beamsplitter 60, here designed for verticallypolarized light. Here, the dichroic beamsplitter is designed to reflectlight having a wavelength of, here 860 nm and transmit light having awavelength of, here 810 nm. Thus, the dichroic beamsplitter 60 isdesigned to reflect the higher, or longest of these two wavelengths andto transmit light having the shortest of these two wavelengths.Therefore, the dichroic beamsplitter 60 reflects the longer wavelength,vertically polarized light from the primary communication lasertransmitter 26b through spacer 174 to a half wave plate 142 forconversion from vertical polarization to horizontal polarization. Thehorizontally polarized light passes through a polarization filter 166 toa polarization beamsplitter 54. The polarization beamsplitter 54transmits the horizontally polarized light to a quarter wave plate 148for conversion to right hand circular polarization. The right handcircularly polarized light is directed by the beam director 33 to areceiver external to the satellite.

The horizontally polarized light from the redundant communication lasertransmitter 26a passes through the bandpass filter 160 and coursealignment wedge 84 to a polarization beamsplitter 58. (Unlike the lightfrom the primary laser transmitter, the light from the redundantcommunication laser transmitter does not pass through a half wave plateto the polarization beamsplitter; thus, the horizontally polarized lightof the redundant communication transmitting laser remains horizontallypolarized). The polarization beamsplitter 58 transmits the horizontallypolarized light to the liquid crystal polarization rotator 128. In a lowvoltage state, the liquid crystal polarization rotator 128 rotates thehorizontally polarized light 90 degrees and thereby converts thehorizontally polarized light to vertical polarization. The verticallypolarized light passes through Y-axis and X-axis alignment liquidcrystal devices 116, 118 to the dichroic beamsplitter 60, here, as notedabove designed for vertically polarized light, to reflect suchvertically polarized light if such light has the higher of twowavelengths. Here, as noted above, the wavelength is 860 nm. Thereflected, vertically polarized light passes through spacer 174 to ahalf wave plate 142 for conversion from vertical polarization tohorizontal polarization. The horizontally polarized light passes througha polarization filter 166 to a polarization beamsplitter 54. Thepolarization beamsplitter transmits the horizontally polarized light toa quarter wave plate 148 for conversion to right hand circularpolarization. The right hand circularly polarized light is directed bythe beam director 33 to a receiver external to the satellite.

The horizontally polarized light from the primary tracking lasertransmitter 24b passes through a bandpass filter 154 to a half waveplate 144 for conversion to vertical polarization. The verticallypolarized light passes through course alignment wedge 78 and is thenreflected by the folding mirror 66 to the polarization beamsplitter 56.The polarization beamsplitter 56 reflects the vertically polarized lightto the liquid crystal polarization rotator 126. In the high voltagestate, the liquid crystal polarization rotator passes verticallypolarized light through the Y-Z plane and X-Y plane alignment liquidcrystal devices 114, 106 to another polarization beamsplitter 52. Thepolarization beamsplitter reflects the vertically polarized light to aliquid crystal polarization rotator 124 The voltage on the liquidcrystal polarization rotator is high so that the vertically polarizedlight remains vertically polarized. The vertically polarized lightpasses through the spacer 172 to the dichroic beamsplitter 60. Thedichroic beamsplitter 60 transmits the 810 nm wavelength, verticallypolarized light through spacer 174 to the half wave plate 142. The halfwave plate 142 converts the vertically polarized light to horizontallypolarized light. The horizontally polarized light passes through thepolarization filter 166 to a polarization beamsplitter 54. Thepolarization beamsplitter 54 transmits the horizontally polarized lightto a quarter wave plate 148 for conversion to right hand circularpolarization. The right hand circularly polarized light is directed bythe beam director 33 to a receiver external to the satellite.

The horizontally polarized light from the redundant tracking lasertransmitter 24a passes through a bandpass filter 156 and coursealignment wedge 80 to a polarization beamsplitter 56. (Unlike the lightfrom the primary tracking laser transmitter, the light from theredundant tracking laser transmitter does not pass through a half waveplate to the polarization beamsplitter; thus, the horizontally polarizedlight of the redundant tracking transmitting laser remains horizontallypolarized). The polarization beamsplitter 56 transmits the horizontallypolarized light to the liquid crystal polarization rotator 126. In ahigh voltage state, the horizontally polarized light remainshorizontally polarized as it passes through the liquid crystalpolarization rotator 126. The vertically polarized light passes throughthe Y-Z plane and X-Y plane alignment liquid crystal devices 114, 106 toa second polarization beamsplitter 52. The polarization beamsplitter 52reflects the vertically polarized light to a second liquid crystalpolarization rotator 124. In a high voltage state, the verticallypolarized light remains vertically polarized. The vertically polarizedlight is passes through spacer 172 to a dichroic beamsplitter 60. Here,the redundant tracking laser transmitter also has the shorter, 810 nmwavelength. The dichroic beamsplitter 60 transmits the verticallypolarized light through spacer 174 a half wave plate 142 for conversionfrom vertical polarization to horizontal polarization. The horizontallypolarized light passes through a polarization filter 166 to apolarization beamsplitter 54. The polarization beamsplitter 54 transmitsthe horizontally polarized light to a quarter wave plate 148 forconversion to right hand circular polarization. The right handcircularly polarized light is directed by the beam director 33 to areceiver external to the satellite.

The primary and secondary acquisition laser transmitters 22b, 22atransmit light at 810 nm; i.e., the shorter wavelength. Further, theprimary acquisition laser transmitter 22b transmits light with verticalpolarization; while the redundant laser transmitter 22a transmits lightwith horizontal polarization.

Thus, the vertically polarized light transmitted by the primaryacquisition laser transmitter 22b passes through bandpass filter 152 andalignment wedge 76 and is then reflected by folding mirror 64 topolarization beamsplitter 50. The vertically polarized light isreflected by the polarization beamsplitter 50 to a liquid crystalpolarization rotator 122. In a low voltage state, the liquid crystalpolarization rotator 122 converts the vertically polarized light tohorizontal polarization. The horizontally polarized light is transmittedby a second polarization beamsplitter 52 to a second liquid crystalpolarization rotator 124. In a high voltage state, the horizontallypolarized light remains horizontally polarized as it passes through theliquid crystal polarization rotator 124. The shorter wavelength,horizontally polarized light passes through spacer 172 to dichroicbeamsplitter 60 and is then transmitted by the dichroic beamsplitter 60,through spacer 174, to a half wave plate 142. The half wave plate 142converts the horizontally polarized light to vertical polarization. Thevertically polarized light is transmitted through polarization filter166 to polarization beamsplitter 54. The polarization beamsplitter 54transmits the vertically polarized light, to quarter wave plate 148 forconversion to right hand circular polarization. The right handcircularly polarized light is directed by the beam director 33 to areceiver external to the satellite.

The horizontally polarized light transmitted by the redundantacquisition laser transmitter 22a is transmitted through a bandpassfilter 150 and alignment wedge 74 to a polarization beamsplitter 50. Thehorizontally polarized light is transmitted by the polarizationbeamsplitter 50 to a liquid crystal polarization rotator 122. In a highvoltage state, horizontally polarized light remains horizontallypolarized as it passes through the liquid crystal polarization rotator122. The horizontally polarized light is transmitted by polarizationbeamsplitter to a second liquid crystal polarization rotator 124. In ahigh voltage state, the horizontally polarized light remainshorizontally polarized as it passes through the liquid crystalpolarization rotator 124. The shorter wavelength, horizontally polarizedlight is transmitted through spacer 172 to a dichroic beamsplitter 60.The light is transmitted through the dichroic beamsplitter 60, throughspacer 174, to a half wave plate 142. The half wave plate 142 convertsthe horizontally polarized light to vertical polarization. Thevertically polarized light is transmitted through the polarizationfilter 166 and polarization beamsplitter 54 to quarter wave plate 148for conversion to right hand circular polarization. The right handcircularly polarized light is directed by the beam director 33 to areceiver external to the satellite.

Considering now received light, during he acquisition and trackingmodes, left hand circularly polarized light received from a sourceexternal to the satellite is directed by the beam director 33 to thequarter wave plate 148 for conversion to vertically polarized light. Thevertically polarized light is reflected by the polarization beamsplitter54, through polarization filter 168 (to purify the verticalpolarization), to liquid crystal polarization rotator 130. It should benoted that the liquid crystal polarization rotator 130 in the path ofthe received light is used to attenuate light from the sun, or incomingsignals from another satellite. Thus, if either one of the detectors 28or 30 tend to saturate, a feedback signal is developed by a processor131 fed by the outputs of the detectors 28, 30 to decrease the voltageon the liquid crystal polarization rotator 130 via line 140. This, tendsto rotate the polarization, which causes attenuation by the polarizationfilter 170. The light then passes to the dichroic beamsplitter 62.During the acquisition and tracking modes, the light used has awavelength of here, 780 nm. Light used for communications here has awavelength of 830 nm. Thus, the acquisition and tracking light have theshorter of the two wavelengths and is transmitted by the dichroicbeamsplitter 62, through course alignment wedge 86 and bandpass filter162, to acquisition/tracking detector 28.

Likewise, left hand circular polarized light received by the satellitewith communication information is also converted to vertically polarizedlight by the quarter wave plate 148 and is reflected by the polarizationbeamsplitter 54, through the polarization filter 168, to the liquidcrystal polarization rotator 130. Here again the control signal on line140 from processor 131 provides a feedback system to attenuate lightfrom the sun, or from another satellite, which tends to saturate eitherone of the detectors 28, 30 in the manner described above. The lightthen passes to the dichroic beamsplitter 62. Because the light used forcommunications here has a wavelength of 830 nm, i.e., the longerwavelength as compared to the wavelength of the light received foracquisition and tracking, the 830 nm wavelength, vertically polarizedlight is reflected by the dichroic beamsplitter 62, through spacer 72,to folding mirror 70. The light is reflected by the folding mirror,through course alignment wedge 88 and bandpass filter 164, tocommunication detector 30.

The bandpass filters 150-164 are centered at the wavelength of the laserlight to be passed by such filters and are used in both the transmit andreceive channels and to provide channel-to-channel and transmit-receiveisolation (i.e., reject all other, unwanted, optical wavelengths). Asnoted from the above description, the polarization beamsplitters 50, 56,and 58 are used as primary and redundant channel combiners. Dichroicbeamsplitters 60 and 62 are used for wavelength division multiplexing onboth the transmit and the receive side of the monolithic opticalstructure. Electro-optic liquid crystal polarization devices, 122, 124,126, 128 and 130 or phase retarders, are used as polarization rotatorsfor redundancy implementation, and as a strong signal attenuator in thereceive path. Electro-optic liquid crystal wedges 106, 108, 114 and 116are used as precision X-Y plane, Y-Z plane beam deflectors foralignment. More particularly, with the liquid crystal wedges 106, 108,114, 116 the nematic liquid crystals are in a wedge shaped structure(sandwitched between a pair of plates having planar outer surfaces) sothat when a voltage is applied to the liquid crystal the correspondingchange in index of refraction causes a deflection in the beam of lightpassing through the wedge. The X-Y plane alignment wedges 106, 108deflects the beam in the X-Y plane, and the Y-Z plane alignment wedges114, 116 deflects the beam in the Y-Z plane. The degree of deflection iscontrolled by the level of the voltage fed to the wedges 106, 108, 114,116 established during an initial alignment process and which aremaintained during normal operation.

The polarization beamsplitters 50, 52, 54, 56, 58 include of a pair ofright angle fused silica prisms cemented together along the hypotenuse,with an embedded multilayer dielectric thin film beamsplitter coating.The dichroic beamsplitters 60, 62 are made of two cemented right-angleprisms with the hypotenuse of one prism coated. The incident light isperpendicular to one face of the cube and the transmitted light exitsthrough the opposite face. The reflected light makes a 90° angle withthe incident light and exits through a side face. The dichroicbeamsplitters 60, 62, are designed to be selective for s-polarizedlight. The dichroic beamsplitters 60, 62 are, as discussed above, usedfor multiplexing the acquisition/tracking and communication channels onthe transmit side and demultiplexing the acquisition/tracking andcommunication channels on the receive side of the terminal. Therefore,there are two dichroic beamsplitters 60, 62, as discussed. A 810/860 nmdichroic beamsplitter 60 for the laser transmitters 22, 24, 26, and an780/830 nm dichroic beamsplitter 62 for the detector/receivers 28, 30,as discussed above.

As noted above, liquid crystal polarization rotators 122, 124, 126, 128,130 are used as voltage-controlled electro-optic polarization rotators.Retarders, also called waveplates, are optical devices that divide alight wave into two orthogonal vector components and produce a phaseshift between these two components. The components recombine on leavingthe device to give a light wave generally of a different polarizationform, as discussed above. The liquid crystal polarization rotators, orretarders, rotate p-polarized light into s-polarized and vice versa, forvernier polarization rotation, redundancy switching, path-switching (asfor rotators 124, 126, 128, and as a voltage-controlled receiveintensity attenuator (as for rotator 130). The nematic liquid crystalcells used are polarization rotators with an electrically adjustableretardance (phase shift). The retardance can be adjusted by applying a 2kHz square wave ac voltage to the liquid crystal. The retardancedecreases as the amplitude of the applied voltage increases. Anamplitude of only a couple volts is required.

Referring now to FIG. 3, an exemplary one of the laser transmittermodule 38 is shown. As noted above, the module 38 includes a pair ofsubmodular units 40, 42. Submodular unit 40 being shown in FIGS. 4, 5,and 5A. Each one of the submodular units 40 includes a corresponding oneof the laser transmitters 22a, 22b, 24a, 24b, 26a, 26b, here lasertransmitter 22a being shown in FIGS. 3, 4 and 5A, a beam shaping lens 41and an optical window 39, as shown. Submodular unit 42 has thecollimating lens 44, as shown. The first and second submodular units 40,42 are aligned with, and affixed to, each other to provide thecollimating/beam shaping module 38. The collimating/beam shaping module38 is then affixed, here bonded with optical cement, to the monolithicoptical structure 34. A corresponding one of the bandpass filters150-160, here bandpass filter 150 is disposed on an surface of thesecond submodular unit 42. The filter 150 protrudes beyond the secondsubmodular unit 42 and is provided with a surface adapted to interface,and be affixed to, a surface portion of the monolithic optical structure34, here to course alignment wedge 74.

The laser transmitter module 38 is adapted to provide a beam ofcollimated light to the diffraction limit. Here, the module 38 is aboutone cubic inch in volume. The submodular unit 40, includes a lasersubmount 200 for securing the laser transmitter 22a, here asemiconductor laser chip. (The laser transmitters are diode lasers herewith 3-5 watts for acquisition and 150 milli-Watts for communicationsand tracking). The multi-Watt acquisition laser transmitters 22a, 22bare broad area devices that are not diffraction limited, but capable offlooding a 1-2 milliradian acquisition field of view. A microlens 41 isbonded very close to the laser emitting facet, here 1 to 4 mils, by anepoxy 202, as shown in FIG. 5. The microlens 41 is aligned with thelaser 22a using multi-axis micro-positioning translation stages. Themicrolens 41 is a aspheric rod microlens for anamorphic correction ofthe laser beam. A thermistor 210 is bonded to the upper surface of thelaser submount 200, as shown. The laser submount 200 is mounted on athermoelectric cooler 214, as shown. The bottom surface of the cooler214 is disposed on a heat transfer device 216, here a molybdenum heattransfer slug, as shown. An alumina substrate 218, here 25 mils thick,is provided to support hybrid electronic driver circuitry 220 for thelaser transmitter 22a. More particularly, in the case of thecommunication laser transmitter, information signals received by thesatellite from the ground station, or the other satellite in the crosslink are fed to the module via an input line 224. (In the case of theacquisition and tracking laser transmitters 22, 24 the signals timemultiplex the operation of these laser transmitters 22, 24 to enablesuccessive, non-concurrent operation. Such signals are provided by aconventional control circuit, here included in the hybrid drivercircuitry 220 to modulate the laser in accordance with signals from theground station 12 or satellite 18, FIG. 1. More particular, theinformation line 224 passes through a feedthrough 226 provided in thelower section 228 of a two piece hermetically sealed package 230, heremade of Kovar material (Ni--Co--Fe), as shown. The alumina substrate 218also has disposed on the upper surface thereof a laser energy detector230, as shown. The detector 230 is disposed under laser submount 200 toreceive a small fraction of the laser energy from the rear facet of thelaser transmitter; the dominant portion of the laser produced energypassing upwardly, in FIG. 3, through the beam forming microlens 41,through a sapphire window 39 mounted to submodular units 40. The twosections 228, 232 of the Kovar package, or submodular units 40 arebonded together to form the submodular units 40, here by solder. Thebottom surface of the alumina substrate 218 is also disposed on the heattransfer device 216, as shown. One method for installing the sapphirewindow 39 is with a borosilicate glass preform (annulus), not shown,having a moderate melting temperature that permits installation of thesapphire window 41 without degrading its optical quality. The fusiblematerial, not shown, should melt at a temperature far below thesoftening temperature of the window and should match the thermalexpansion coefficient of both the sapphire window 39 and the submodularunits 40 package material. This method permits sealing of the sapphirewindow 39 into the package 40 without introducing mounting wavefrontdistortion into the window 39.

Preferably, the laser beam produced by the laser transmitter 22a shouldexit the laser transmitter 22a through the beam shaping microlens 41with less than 1 milliradian deviation from the perpendicularity withrespect to the mechanical axis of the module 38.

The laser transmitter 22a is be cooled by the cooler 214 to pull itsnominal as-procured wavelength down to its required wavelength. This, inturn, requires a hermetically sealed package 40 filled with an inert gasin order to prevent condensation on the laser facet. The thermistor 210measures the temperature of the laser transmitter 22a an provides afeedback control signal to the cooler 214, to provide proper temperaturecontrol for the laser transmitter 22a. The Kovar material preferablyused for the package 40 has a thermal expansion coefficient (5.3×10⁻⁶/degree C.) to match the thermal expansion coefficient of theborosilicate glass and also to match the thermal expansion coefficientof the alumina substrate 218 (6.7×10⁻⁶ /degree C.). It also matches thethermal coefficient of expansion of the sapphire window 39. Here, theKovar package 40 has a molybdenum heat sink slug, as noted above, toprovide a low thermal resistance to heat sink 219. Molybdenum ispreferred because it has the same thermal expansion coefficient as theKovar package 40, but significantly better thermal conductivity (140versus 17 W/m-degrees C.) to efficiently couple the heat out of thethermal cooler 204 and into the heat sink 219 via the heat transferdevice 216. An alternative to the use of the molybdenum thermal slug isa metal matrix packaging material, such as SILVAR material developed byTexas Instruments, machined or stamped, and attached with epoxy, solderor braze, may be used. Such material permits the use of fused glassseals, such material also has a high thermal conductivity comparable tomolybdenum (157 versus 140 W/m-degrees C.), thereby eliminating the needfor a separate thermal slug for the package base 216. As noted above,the thermistor 210 and thermoelectric cooler 214 are provided for lasertransmitter 22a operating wavelength and output power stabilization bytemperature control. Also provided is a hybrid laser driver circuit 220,here adapted to provide 200-300 milliamps at modulation rates of up to155 Mbps. The base 216 of the submodular units 40 conducts heat from thelower, warm side of the thermoelectric cooler 214 and the hybrid drivercircuitry 220 to heat sink 219 via the base, or heat transfer device216. A flexible thermal interface 217 having a high thermal conductivityis provided between the heat transfer device 216 and the heat sink 219,as shown, to accommodate differences in thermal expansion between theheat transfer device 216 and the heat sink 219). The flexible interface217 preferable is a thermal tape with a binder, a silicone-based thermalgrease, or a conductive adhesive with the appropriate thermalproperties.

The laser transmitter metal package, or submodular units 40, is bondedto submodular units 42. More particularly, the submodular units 40 isbonded onto one end of a collimating lens sleeve 240. The lens sleeve240 provides a holder for the collimating lens 44 and as a mechanicalinterface for submodular unit 40. The use of two submodular units 40, 42for the laser transmitter module 38 splits the laser collimating lens 44optics and the laser transmitter with its microlens beam shaping lens 41on its facet optics and thereby provides for two separately alignedunits (i.e, submodular unit 40 and submodular unit 42). The use of twoseparate aligned units, or submodular units 40, 42 relaxes thecollimating lens 44 alignment tolerances, as compared to the use of asingle module having both the collimating lens 44 and the lasertransmitter with its microlens 39 on its facet, because the f/number ofthe beam into the collimating lens 44 is, typically on the order off/2.5 rather than the f/1.0 beam that exits the laser transmitter facet.

Preferably, the lens sleeve 240 is an opaque ceramic, a compositematerial or a metal matrix with low thermal coefficient of expansion sothat thermal excursions will not induce a radial tensile strain into thecollimating lens 44 or the bandpass filter 150 (FIG. 2) bonded to theceramic sleeve 240. Material for the sleeve 240 preferably are an Invarmaterial or graphite composite (less than or equal to 0.2×10⁻⁶ /degreesC.) or carbide-machinable ceramic (9.4×10⁻⁶ /degrees C.) , such as Macormaterial from Corning, depending on the desired expected thermalvariation.

Referring now to FIG. 6, the other satellite 18 (FIG. 1) linked with thesatellite 16, is shown at a first instant in time t1 and at a somewhatlater instant in time t2. The laser energy transmitted by theacquisition/tracking lasers, not shown, in satellite 18, indicated byarrow 90, is directed by the fine steering mirror 31 to theacquisition/tracking detector 28 of satellite 16, through the monolithicoptical structure 34. (It is noted that the monolithic optical structure34 is only diagrammatically represented in FIG. 6). The light receivedby satellite 16 from satellite 18 at time t1 is directed to the detector28. Here, detector 28 uses a charge coupled device having an array ofrows and columns of detector pixels, as shown, in FIG. 8. As shown bythe "o" in FIG. 8, the received light is here shown focused to a pixelhaving an X, Y coordinate of Xn, Ym. The center, or boresight, or opticaxis is indicated by the "x" in FIG. 8. Because of the relative motionbetween satellite 16 and satellite 18, here indicated by arrow 92 inFIG. 6, it is necessary that the beam of laser energy transmitted bysatellite 16, indicated by arrow 94, "lead", or point ahead of, thelight (arrow 90) transmitted by the satellite 18 by a lead angle, L. Acontrol system 96 (FIG. 6) responds to the pixel in detector 28 (FIG.8), here pixel Xn, Ym receiving the focus light from satellite 18 andproducing a boresight tracking error, in a conventional feedback controlsystem tracking loop 98, as shown in FIG. 7. Here, however, instead ofhaving the tracking loop 98 drive the fine steering mirror 31 to nullthe boresight error signal and thereby drive the boresight, or opticaxis of the optical system to point at the satellite 18, i.e., point inthe direction of the light 90 transmitted by satellite 18, the requiredlead angle, (as computed by computer 100, in a conventional manner usingconventional geometric equations) is added to the tracking signalproduced by the tracking loop 98 with the result that the optical axisof the optical system is directed to the expected position of satellite18 at the subsequent time t2, as shown in FIG. 6. Because theacquisition, tracking and communication lasers are all aligned with theoptic axis, the laser beams produced by such laser will be directed tothe satellite 18 at its expected position at time t2. Thus, the trackingloop 98 tracks with a finite boresight error signal, i.e., the leadangle, L. With this arrangement, a single mirror, here fine steeringmirror 31, is used during all phases (i.e., the acquisition, trackingand communication phases). That is, by using a spatially resolveddetector (i.e., here a CCD device, which provides a signalrepresentative of the actual position of the received light energyrelative to the boresight, or optic axis) the tracking loop 98 is ableto maintain the focused energy at a fixed lead angle, L, off of theboresight, or optic axis, as shown in FIG. 8, thereby eliminating theneed for a separate "point ahead" mechanism for the laser transmitters.

To put it another way, the monolithic optical structure 34 (FIG. 6)provides an interface between the beam director, here fine beam steeringmirror 31 thereof and the laser transmitter 22, 24, 26 and laser energydetector 28. The monolithic optical structure has an optic axis, orboresight axis, passing between the beam director, here mirror 34thereof, and the laser transmitter 22, 24, 26 and passing between thebeam director/mirror 34 and the laser energy detector 28. Incomingenergy from a source of laser energy, satellite 18, moving relative tothe tracking, or control system 96 is directed by beam director/mirror31 and the optical system 34 to the laser energy detector 28 along theoptic axis and the laser energy being produced by the laser transmitter22, 24, 26 is directed through the optical structure 34 and the beamdirector/mirror 31 along the optic axis to the source 18. The monolithicoptical structure 34 directs the incoming laser energy to a position onthe laser energy detector 28 (i.e., the focal plane of the CCD array ofpixel, FIG. 8) related to the angular deviation between the direction ofthe optic axis and the direction of the incoming laser energy. Computer100 (FIG. 7) computes the lead angle, L, between the present directionto the source 18 (i.e., the direction 101 (FIG. 6) between satellite 16and satellite 18 at time t1) and an expected direction to the source 18(i.e., the direction 103 between satellite 16 and satellite 18 at timet2. The control system 96 (FIG. 7) is responsive to a signal producedthe laser energy detector 28 related to the position of the incominglaser energy on such detector relative to the optic axis (FIG. 8) andthe computed lead angle, L, for tracking the source of incoming laserenergy with a tracking error related to the computed lead angle anddirecting the optic axis along the expected direction 103 (FIG. 6) tothe source 18.

Here, the acquisition field of view is 1 milliradian. Here a 256×256pixel detector permits tracking accuracy on the order of 1 micro radian.If the ratio of acquisition field of view to tracking accuracy is on theorder of 1000 to 1, then the same detector may be usable for both theacquisition and tracking modes. In such case, the tracking loop 98 isadapted to have a selected one of two bandwidths; a slower responding(I.e., smaller) bandwidth during the acquisition mode and a largerbandwidth during the tracking mode. Such dual mode operation isrepresented diagrammatically in the tracking loop by a pair ofamplifier-shaping networks G1, G2, respectively; amplifier-shapingnetwork G1 being switched into the loop 98 during the acquisition modeand amplifier-network G2 being switched into the loop 98 during thetracking mode.

Other embodiments are within the spirit and scope of the appended claim.For example, while the system has been shown for use withsatellite-satellite cross links and ground station up-links, the lasercommunication system could be used in satellites which provide groundsurveillance information developed by instrumentation carried on-boardthe satellite.

What is claimed is:
 1. A laser communication system adapted for use in asatellite, such laser communication system comprising:a beam director;laser transmitter/receivers; a monolithic structure comprising aplurality of active and passive optical elements for interfacing betweenthe beam director and the laser transmitters/receivers, laser energybeing directed between the beam director and the transmitters/receiversby the active and active optical elements, wherein the monolithicstructure is configured to provide all optic axes between the beamdirector and laser transmitter/receivers in substantially a commonplane.
 2. The laser communication system recited in claim 1 wherein thelaser transmitter/receivers include an acquisition laser transmitter andan acquisition receiver used to enable the satellite to link up withanother satellite, or ground station, during an acquisition mode, and acommunication laser transmitter and an communication receiver adapted toenable the satellite to exchange data with the linked up satellite, orground station during a communication mode.
 3. The laser communicationsystem recited in claim 2 wherein the monolithic structure is configuredto dispose the optic axis between the beam director and laseracquisition and communication lasers and the optic axes between the beamdirector and the acquisition and communication receivers insubstantially a common plane.
 4. The laser communication system recitedin claim 3 wherein the laser transmitter/receivers include: a trackinglaser transmitter for enabling the linked up satellite, or groundstation to track the satellite during a tracking mode; and a singledetector use by the satellite during both the acquisition mode and asubsequent tracking mode.
 5. The laser communication system recited inclaim 4 wherein the single detector includes a charge coupled device. 6.A laser communication system adapted for use in a satellite, such lasercommunication system comprising:a beam director; lasertransmitter/receivers; a monolithic structure comprising a plurality ofactive and passive optical elements for interfacing between the beamdirector and the laser transmitters/receivers, laser energy beingdirected between the beam director and the transmitters/receivers by theactive and active optical elements, wherein the monolithic structure isconfigured to provide all optic axis between the beam director and thelaser transmitter/receivers in a substantially common plane; andincludinga collimating/beam shaping module, such module comprising apair of submodular units, a first one of the submodular unit includingone of the transmitting lasers and a beam shaping lens, the second oneof the submodular units having a collimating lens, the first and secondsubmodular units being aligned with, and affixed to, each other toprovide the collimating/beam shaping module, the collimating/beamshaping module being affixed to the monolithic structure.
 7. The lasercommunication system recited in claim 6 including a collimating/beamshaping module filter disposed on an surface of the second submodularunit.
 8. The laser communication system recited in claim 7 wherein thefilter protrudes beyond the second submodular unit and is provided witha surface adapted to interface, and be affixed to, a surface portion ofthe monolithic structure.
 9. A method of assembling a lasercommunication system adapted for use in a satellite, said lasercommunication system comprising: a beam director; lasertransmitter/receivers; and a monolithic structure comprising a pluralityof active and passive optical elements for interfacing between the beamdirector and the laser transmitters/receivers, laser energy beingdirected between the beam director and the transmitters/receivers by theactive and active optical elements, such method comprising the stepsof:forming a first submodular unit, said unit comprising one of thelasers and a properly aligned beam shaping lens; forming a secondsubmodular unit, said second unit having a mounted collimating lens;aligning the first and second submodular units with each other to form acollimating/beam shaping module; and affixing the collimating/beamshaping module to a surface of the monolithic structure.
 10. The methodrecited in claim 9 including the step of providing a filter on ansurface of the second submodular unit.
 11. The method recited in claim10 wherein the filter protrudes beyond the second submodular unit and isprovided with a surface adapted to interface, and be affixed to, thesurface of the monolithic structure.
 12. A laser communication systemadapted for use in a satellite, such laser communication systemcomprising:a transmitting laser, responsive to electrical signals forconverting such electrical signals into corresponding light signals; adetector adapted to receive light signals transmitted by a laser andconvert such light signals into corresponding electrical signals; a beamdirector adapted to direct light signals transmitted by the satellite toa receiver external to the satellite and to direct light signalsreceived by the satellite from a source external to the satellite; and,a monolithic optical structure, for passing therethrough the lightsignals from the transmitting laser to the beam director and for passingtherethrough light signals received by the beam director to thedetector, such monolithic optical structure comprising a plurality ofglass cubes and planar thin film bonded together; and wherein themonolithic structure is configured to provide all optic axis between thebeam director and the laser transmitter/receivers in a substantiallycommon plane.
 13. The laser communication system recited in claim 12including, additionally, a redundant laser transmitter and wherein thethin films are responsive to electrical control signals to direct thelight signals from either the first mentioned laser transmitter or theredundant laser transmitter through the monolithic optical structure tothe beam director selectively in accordance with the control signals.14. The laser communication system recited in claim 13 including ansecond pair of laser transmitters, and wherein the thin films areresponsive to electrical control signals to direct the light signalsfrom either a first one of the second pair of laser transmitters or asecond one of the second pair of laser transmitters thorough themonolithic optical structure to the beam director selectively inaccordance with the control signals.
 15. The laser communication systemrecited in claim 14 including a third pair of laser transmitters, andwherein the thin films are responsive to electrical control signals todirect the light signals from either a first one of the third pair oflaser transmitters or a second one of the third pair of lasertransmitters thorough the monolithic optical structure to the beamdirector selectively in accordance with the control signals.
 16. Thelaser communication system recited in claim 15 wherein one of the threepairs of laser transmitters is used during an acquisition mode, a secondone of the three pairs of laser transmitters is used during a trackingmode, and a third one of the three pairs of laser transmitters is usedfor communication of data during the tracking mode.
 17. A lasercommunication system adapted for use in a satellite, such lasercommunication system comprising:a transmitting laser, responsive toelectrical signals for converting such electrical signals intocorresponding light signals; a detector adapted to receive light signalstransmitted by a laser and convert such light signals into correspondingelectrical signals; a beam director adapted to direct light signalstransmitted by the satellite to a receiver external to the satellite andto direct light signals received by the satellite from a source externalto the satellite; and, a monolithic optical structure, for passingtherethrough the light signals from the transmitting laser to the beamdirector and for passing therethrough light signals received by the beamdirector to the detector, such monolithic optical structure comprising aplurality of glass cubes and planar thin film bonded together; aredundant laser transmitter and wherein the thin films are responsive toelectrical control signals to direct the light signals from either thefirst mentioned laser transmitter or the redundant laser transmitterthrough the monolithic optical structure to the beam directorselectively in accordance with the control signals; a second pair oflaser transmitters; wherein the thin films are responsive to electricalcontrol signals to direct the light signals from either a first one ofthe second pair of laser transmitters or a second one of the second pairof laser transmitters thorough the monolithic optical structure to thebeam director selectively in accordance with the control signals; athird pair of laser transmitters, and wherein the thin films areresponsive to electrical control signals to direct the light signalsfrom either a first one of the third pair of laser transmitters or asecond one of the third pair of laser transmitters thorough themonolithic optical structure to the beam director selectively inaccordance with the control signals; wherein one of the three pairs oflaser transmitters is used during an acquisition mode, a second one ofthe three pairs of laser transmitters is used during a tracking mode,and a third one of the three pairs of laser transmitters is used forcommunication of data during the tracking mode; and including: anadditional detector, and wherein the thin films are responsive toelectrical control signals to attenuate light signals from the beamdirector to either one of the detectors selectively in accordance withthe control signals developed by the detectors and coupled to the thinfilms in a feedback loop.
 18. The laser communication system recited inclaim 17 wherein optical paths between the three pairs of transmittinglasers and the beam director and between the beam director and the pairof detectors are disposed in substantially a common plane.
 19. The lasercommunication system recited in claim 18 wherein light signalstransmitted by the transmitting laser in one of the three pairs thereofand the light signals transmitted by the transmitting laser in a secondone of the three pairs are at different wavelengths.
 20. The lasercommunication system recited in claim 19 wherein laser signals passedfrom the beam director to a first one of the pair of detectors andlasers signals passed from the beam director to a second one of the pairof beam director have different wavelengths.
 21. The laser communicationsystem recited in claim 20 wherein the optical paths of two of the threepairs of transmitting lasers pass through a portion of the monolithicstructure in the same direction, wherein the optical paths of the thirdpair of transmitting lasers pass through one portion of the monolithicstructure in a direction perpendicular to the aforementioned direction.22. A laser communication system adapted for use in a satellite, suchlaser communication system comprising:a beam director; lasertransmitter/receivers; a monolithic structure comprising a plurality ofactive and passive optical elements for interfacing between the beamdirector and the laser transmitters/receivers, laser energy beingdirected between the beam director and the transmitters/receivers by theactive and active optical elements, wherein the monolithic structure isconfigured to provide all optic axes between the beam director and lasertransmitter/receivers in substantially a common plane; and including abeam shaping/collimating laser transmitter module attached to themonolithic optical structure, such laser transmitter module comprising:afirst submodular unit having a laser transmitter of the lasertransmitter/receivers and a beam shaping lens affixed to such lasertransmitter; a second submodular unit having affixed thereto acollimating lens for collimating a beam produced by the lasertransmitter; wherein the first and second submodular units are alignedwith, and affixed to, each other to provide the collimating/beam shapingmodule.
 23. The system recited in claim 22 including a collimating/beamshaping module filter disposed on an surface of the second submodularunit, such filter being provided with a surface adapted to interface,and be affixed to, a surface portion of the monolithic opticalstructure.
 24. The system recited in claim 23 wherein the lasertransmitter module is adapted to provide a beam of collimated light tothe diffraction limit.
 25. The system recited in claim 23 the firstsubmodular unit includes a submount for the laser transmitter.
 26. Thesystem recited in claim 25 wherein a microlens is affixed to thesubmount.
 27. The system recited in claim 26 wherein the beam shapinglens is a microlens bonded in close proximity to a laser emitting facet.28. The system recited in claim 27 wherein the microlens is a asphericrod microlens for anamorphic correction of the laser beam.
 29. Thesystem recited in claim 28 including a thermistor bonded to the uppersurface of the submount.
 30. The system recited in claim 29 includes alaser energy detector disposed to receive a small fraction of the laserenergy produced by the laser transmitter passing through the opening tosuch detector.
 31. The system recited in claim 30 including athermoelectric cooler disposed between the laser transmitter and a heattransfer device.
 32. The system recited in claim 31 including asubstrate and electronic driver circuitry supported on the substrate andelectrically coupled to the laser transmitter.
 33. The system recited inclaim 32 wherein the first modular unit comprises a hermetically sealedpackage.
 34. The system recited in claim 33 wherein the substrate hasdisposed on an upper surface thereof the laser energy detector.
 35. Thesystem recited in claim 34 wherein the first submodular unit includes asapphire window mounted to the package.
 36. The system recited in claim34 wherein the first modular unit is filled with an inert gas.
 37. Thesystem recited in claim 33 wherein the thermistor measures thetemperature of the laser transmitter and provides a feedback controlsignal to the cooler to provide temperature control for the lasertransmitter.
 38. The system recited in claim 37 wherein material usedfor the package has a thermal expansion coefficient matched to thethermal expansion coefficient of the substrate.
 39. The system recitedin claim 37 wherein the material used for the package has a thermalexpansion coefficient matched to the thermal expansion coefficient ofthe sapphire window.
 40. A laser communication system adapted for use ina satellite, such laser communication system comprising:a beam director;laser transmitter/receivers; a monolithic structure comprising aplurality of active and passive optical elements for interfacing betweenthe beam director and the laser transmitters/receivers, laser energybeing directed between the beam director and the transmitters/receiversby the active and active optical elements, wherein the monolithicstructure is configured to provide all optic axes between the beamdirector and laser transmitter/receivers in substantially a commonplane; and wherein the transmitter/detectors include a single detectorfor an acquisition mode and a tracking mode and wherein thecommunication system includes a control system, fed by the detector, toproduce control signals for positioning the beam director.
 41. Thesystem recited in claim 40 wherein the control system includes atracking loop fed by the single detector and a lead angle computer fordriving the optic axis of the monolithic optical structure to apredetermined tracking error, such tracking error being related to alead angle computed by the lead angle computer.